1. Field of the Invention
The present invention relates to solutions of polyimide-forming substances, containing
A) aromatic or partly aromatic diamines and PA1 B) tetraesters of imide-forming aromatic or partly aromatic tetracarboxylic acids or mixtures of these tetraesters with the corresponding tri-, di- and/or monoesters.
The present invention furthermore relates to the use of such solutions for the production of coatings.
Polyimides are being increasingly used for coating metal wires, glass and quartz fibers and silicon substrates (silicon wafers). They are used in particular in electronics and microelectronics, for example as intermediate or top layers in the production of circuits.
The substrates are generally coated with solutions of polyimides or polyamic acids, conversion to the imide taking place on the substrate surface in the case of the polyamic acid.
The solutions of polyimides and polyamic acids must, however, be of a relatively low concentration in order to give a viscosity sufficiently low for processing. Typical commercial products generally have a solids content of only from 12 to 20% by weight.
The low concentration results in high shrinkage during drying. This shrinkage leads to a greater or lesser impression of the structure under the polyimide layer in the polyimide surface.
Inaccuracies in the structure occur especially in the case of, for example, multilayer coatings required for circuits.
For this reason and to avoid excessive amounts of solvents, more highly concentrated solutions are desirable. Higher concentrations are possible in principle with solutions which contain only the starting materials for the polyimides. However, since solutions of tetracarboxylic dianhydrides and diamines do not have a long shelf life but polymerize to give polyamic acid, diesters of tetracarboxylic acids have been used to date instead of the tetracarboxylic dianhydrides.
2. Description of the Related Art
U.S. Pat. No. 3,700,649 describes solutions of diesters of benzophenonetetracarboxylic acid and diamines having a very low basicity for the production of polyimide coatings. Alkanols and benzyl alcohol are mentioned as alcohols for the esterification.
In the case of the solutions of diesters of tetracarboxylic acids and diamines, disclosed in U.S. Pat. No. 3,347,808, alkanols are used as the esterification component.
U.S. Pat. No. 4,874,835 relates to solutions of diamines and diesters for oxydiphthalic acid with alkanols.
U.S. Pat. No. 4,960,824 discloses solutions of diesters of araliphatic tetracarboxylic acids with alkanols and at least one diamine selected from the group consisting of m-phenylenediamine, p-phenylenediamine and 4-aminophenyl ether.
German Patent Applications P 41 35 056.1 and P 41 35 058.8 describe solutions of polyimide-forming starting materials, such as diamines and diesters of tetracarboxylic acids, and their use for coatings.
According to the prior art, the use of esters other than the diesters is not regarded as helpful. However, a disadvantage of the use of pure diesters is an increase in the viscosity on prolonged storage, presumably due to the formation of polymeric salts. Another disadvantage is the formation of bubbles during the production of layers thicker than about 30 .mu.m. However, such thick layers are essential for buffer layers in chip production.